A full IPv6 header is 40 bytes, and an IEEE 802.15.4 frame carries a payload of roughly a hundred bytes, so sending IP over such radios would waste most of each packet. 6LoWPAN, defined in RFC 4944 and later RFCs from the IETF, elides fields that can be inferred from the link, compresses addresses derived from MAC identifiers, and fragments and reassembles packets larger than one frame.
The payoff is that constrained devices become ordinary IP hosts: they can run UDP, CoAP and DTLS, be addressed from anywhere the network policy permits and be routed through a border router with no application gateway. Thread and Wi-SUN are built on it. The trade-off is that fragmentation over lossy links is fragile, so protocols on top are designed to keep messages small enough to fit in a single frame. See Thread explained.
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